1. Technical Field
The present disclosure relates to a negative electrode and a lithium ion secondary battery.
2. Related Art
Lithium ion secondary batteries are lighter in weight and higher in capacity than nickel-cadmium batteries, nickel-metal hydride batteries, and the like. The lithium ion secondary battery has been therefore widely used as a power source for a mobile electronic appliance. Further, as the mobile electronic appliances decrease in size and increase in functionality in recent years, the lithium ion secondary battery has been expected to have further higher capacity. Not just for the mobile electronic appliances, furthermore, the high-capacity lithium ion secondary battery has been a strong candidate as a power source to be mounted on a hybrid vehicle, an electric vehicle, or the like.
The capacity of the lithium ion secondary battery mainly depends on an active material of an electrode. In general, for example, a negative electrode active material contains graphite. However, for meeting the above demand, a negative electrode active material with higher capacity has been desired. Therefore, silicon has attracted attention as a material for the negative electrode active material. Silicon has mud higher theoretical capacity (4210 mAh/g) than the theoretical capacity (372 mAh/g) of graphite.
In a negative electrode active material including a mixture of silicon and silicon oxide, the stress caused by expansion and shrinkage of silicon during charging and discharging is relaxed by silicon oxide. The mixture is therefore considered to have superior cycle characteristics to silicon. However, the mixture of silicon and silicon oxide has poor electric conductivity. As a result, when the current density during discharging is high relative to the battery capacity, the use of this mixture as the negative electrode active material causes a significant decrease in discharge capacity. On the other hand, the power source for a hybrid vehicle and an electric vehicle is desired to have high discharge capacity at high rate. Therefore, the use of this mixture as the material of the negative electrode active material of such a power source has a problem.
JP-A-2008-277031 has disclosed a technique for increasing the discharge capacity at high rate. A secondary battery according to this technique has a current collector that reversibly intercalates and deintercalates lithium ions. At least one surface of this current collector is provided with a concave part and a convex part. Further, this secondary battery has columnar bodies each formed on the convex pan of the current collector. In this configuration, a space part is formed between the columnar bodies. Therefore, the columnar bodies can intercalate and deintercalate lithium from the entire periphery.